This invention relates generally to plasma etch processes, and more particularly to an interferometric method and apparatus for monitoring a plasma etch process.
In the fabrication of integrated circuits, the removal of various layers or thin films of materials formed on a silicon wafer to define device patterns is commonly accomplished by means of an etching process. Etching techniques in use include wet, or chemical etching, and dry, or plasma etching. The latter technique is typically dependent upon the generation of reactive species from process gases that are impinged on the surface of the material to be etched. A chemical reaction takes place between the material and these species and the gaseous reaction product is then removed from the surface.
An important consideration in all etch processes is control of the extent to which the wafer is etched and determining a time, referred to as the endpoint, at which to end the process. Common methods for monitoring the etch process and determining the endpoint include spectroscopy and interferometry. Interferometric methods known in the prior art include laser interferometry and plasma emission interferometry as disclosed in U.S. Pat. No. 5,450,205 to Sawin et al. In the laser interferometric method, a laser beam I generated by laser 10 is directed through an optical window 12 and onto an area of a wafer 14 undergoing etching within a plasma chamber 16 as shown in FIG. 1. The intensity of the reflected beam R is detected by a detector 18 and recorded as a function of time. The detector may be a bandpass filter coupled with a silicon photodiode, a spectrometer, or a CCD camera.
When the material being etched is relatively transparent to the incident light, such as layer A as shown in FIG. 2, and overlies a reflective surface, such as layer B, the detected light intensity goes through a series of maxima and minima. As layer A is transparent to the incident light, the incident light is both reflected from the upper surface of the layer A and is refracted through the material. At the reflective surface of layer B, the refracted light is also reflected upwardly through layer A, exiting layer A to interfere with the light reflected from the upper surface of layer A. As layer A is etched, the optical path through layer A decreases in length resulting in varying interference patterns.
Plasma emission interferometry also analyzes the interference of light reflected from the surface of a wafer but uses etch reactor plasma optical emission as the light source. As shown in FIG. 3, incident light Ixe2x80x2 generated from plasma emission 20 formed within the plasma chamber 22 is reflected from the surface of a wafer 24 disposed within the chamber 22. The reflected light Rxe2x80x2 from the wafer 24 passes through an optical window 26 and is detected by a detector 28.
A plasma chamber 30 having a top portion 32 formed of a dielectric material transmissive to radiation is shown in FIG. 4. In the case where the dielectric material is transparent, such as fused silica, the top portion 32 can serve as an optical window 33. As shown, a light source 34 provides an incident beam Ixe2x80x3 which illuminates the surface of a wafer 36 through the optical window 33. The reflected light Rxe2x80x3 exits a plasma chamber 38 through the optical window 33 and is detected by detector 39. Although not shown, optical emission generated by the plasma may also be detected by the detector 39 in the case where no light source 34 is used.
A common problem with the prior art systems shown in FIGS. 1 and 3-4 relates to the difficulty in maintaining the optical quality of a window exposed to the plasma. The plasma either etches the window, in which case the window loses its clarity, or the plasma deposits material onto the window, which also leads to a loss of clarity. In the case of optical window 33 shown in FIG. 4, these problems are further exacerbated by the fact that a bottom surface 31 facing the plasma 35 of the top portion 32 is typically roughened. Deposited materials adhere better to roughened surfaces than to smooth surfaces and are less likely to flake off onto the wafer being etched. As a consequence of roughening the bottom surface 31, the optical window 33 becomes translucent rather than transparent and is not very useful for as an optical window for prior art interferometric monitoring methods.
A solution to maintaining the optical quality of a window is disclosed in pending application Ser. No. 09/282,519 to Ni et al. assigned to LAM Research Corporation. With reference to FIG. 5, a plasma chamber 40 includes a radiation transmissive top portion 42 having a recessed optical window 44 formed therethrough. Process gas flows into the plasma chamber 40 through an inlet 45 in communication with a prechamber 46, the prechamber being in communication with the interior of the plasma chamber 40. The flow of process gases prevents the plasma 47 from etching or depositing material on the optical window 44. Interferometry is then performed conventionally using a light source 48 and detector 49.
While the optical window 44 works optically well, it suffers from the disadvantage of increasing the cost of the fused silica dielectric window. In addition to the cost of machining a hole in the dielectric window to accommodate the prechamber 46, the window is structurally weakened by the machining of the hole. As the dielectric window serves as portion of a vacuum chamber, it must be made thicker to restore the loss in structural strength. This further increases the cost of the dielectric window and reduces the effectiveness of the dielectric window in coupling the radiation to the plasma. An additional drawback of the recessed window solution disclosed is that the top center of the plasma chamber is not the optimum location for the process gas feed for all purposes.
It would therefore be desirable to be able to detect interferometric signals from a wafer being etched without incurring the additional costs, shifting the process, or constraining the gas injection as is required by prior art methods of keeping the optical window clean.
The present invention provides an interferometric method and apparatus for monitoring a plasma etch process that interposes a diffusing or scattering element between the wafer and the detector. The diffusing or scattering element eliminates the need for a hard-to-maintain transparent optical window located in the top wall of the chamber. It either replaces the transparent window, or allows the transparent window to be moved from a position in the top wall of the chamber to a position in the side wall of the chamber, wherein the window is less susceptible to being degraded by high-density plasma.
More particularly, the present invention is embodied in a plasma chamber having a top wall formed of fused silica, the top wall having a top surface and a bottom surface facing the interior of the plasma chamber. In a first embodiment of the invention, light generated by plasma emission is reflected from the wafer, is scattered at the bottom surface of the top wall, and is transmitted through the top surface of the top wall. Detecting apparatus comprising a lens, optical fiber and a detector detect the light opposite the top wall from the wafer.
In another embodiment of the invention, a light source is provided for illuminating the wafer. Light from the light source passes through the top surface of the top wall and is diffused at the bottom surface of the top wall. The diffused light from the light source illuminates the wafer and is reflected from the wafer. The light reflected from the wafer illuminates the bottom surface of the top wall, is scattered by that surface and is transmitted through the top surface of the top wall. Detecting apparatus comprising a lens, optical fiber and a detector detect the light opposite the top wall from the wafer.
In another embodiment of the invention, a screen is disposed inside the plasma chamber. Light from plasma emission is reflected from the wafer, scattered from the screen and detected through a viewing window by a detecting apparatus.
In another embodiment of the invention, a screen is disposed inside the plasma chamber and a light source illuminates the wafer through a window. Light reflected from the wafer is scattered by the screen and is detected through a viewing window.
These and other advantages of the present invention will become apparent to those skilled in the art upon a reading of the following description of the invention and a study of the several figures of the drawings.